Denoting the region sandwiched between \({A_i}\)  and \({A_j}\)  as \(\tilde{A} = {A_{\left( {i + 1} \right) \cdots \left( {j - 1} \right)}}\)  (which is a composite region composed of a set of elementary regions), then one can solve (REF ) and obtain [1]},
\({F_{ij}} = \frac{1}{2}\left( {S\left( {\tilde{A} \cup {A_i}} \right) + S\left( {\tilde{A} \cup {A_j}} \right) - S\left( {\tilde{A}} \right) - S\left( {{A_i} \cup \tilde{A} \cup {A_j}} \right)} \right).\) 
